Plating method utilizing silver-coated anode leads

ABSTRACT

IN AN ELECTRODEPOSITION METHOD UTILIZING A CHLORIDE ELECTROLYTE, THE CONDUCTOR TO THE ANODE HAS A BASE OR CORE OF CONDUCTIVE METAL AND A COATING OF SILVER.

United States US. Cl. 204-49 5 Claims ABSTRACT OF THE DISCLOSURE In an electrodeposition method utilizing a chloride electrolyte, the conductor to the anode has a base or core of conductive metal and a coating of silver.

BACKGROUND OF THE INVENTION Although it has been found that certain metals are advantageously electrodeposited from baths which contain chloride ion as the electrolyte, the use of such baths often presents a problem from the standpoint of the conductors which may suitably be used as the lead to the anode in the bath. The presence of chloride ion tends to have a seriously deleterious effect upon the leads conventionally used in plating baths, often rendering them wholly unsuitable for use in chloride plating solutions. For example, leadcoated copper conductors are often employed as leads to the anode in sulfate baths for electrodepositing zinc, but it has been found that they should not be used in chloride electroplating baths because of the level of solubility of lead under the conditions of plating and the resultant contamination of the bath and the coating produced.

Accordingly, it is an object of the present invention to provide a method for electrodepositing metal from a chloride-containing bath in which the anode conductor is relatively inexpensive and is substantially inert to attack so as to avoid contamination of the bath and of the electrodeposited coating therewith.

It is also an object of the invention to provide such a method wherein the lead to the anode is a highly eificient conductor of electricity, despite its inertness to attack, and wherein the lead is relatively inexpensive to produce and has a long operating life.

SUMMARY OF THE INVENTION across the anode and cathode to deposit metal from the solution upon the cathode. The conductor between the anode and the source of DC. potential is at least partially submerged in the solution and consists essentially of an electrically-conductive metal base member and a substantially nonporous coating of silver thereon. The metal of the base member is susceptible to attack by the chloride ion in the plating solution and the coating is present continuously over the entire surface of at least the portion of the conductor which is submerged in the chloride-containing solution.

Preferably, the solution contains zinc ion as the metal for deposition and desirably it also contains ammonium ion. More specifically, the solution may advantageously contain about 10.0 to 250.0 grams per liter of zinc chloride, about 20.0 to 220.0 grams per liter of ammonium chloride and about 5.0 to 15.0 grams per liter of surface active and brightening agents. Most desirably, the metal 3,649,478 Patented Mar. 14, 1972 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the most basic terms, the method of the invention involves the use of silver-coated anode conductors in chloride-containing electroplating baths, which may be of conventional composition. Normally, the baths will contain at least about 10 grams of free chloride ion per liter to be classified as chloride-containing within the scope of this invention, but as a practical matter chloride plating baths contain about 40.0 to 300.0 grams of chloride ion per liter of bath.

Although the method of the invention may be employed to deposit substantially any metal which can be electrodeposited from a chloride bath, it is most beneficially used to deposit zinc; zinc is readily deposited from a chloride bath and is also easily contaminated. Exemplary of the other metals with which the method may be employed are nickel, cadmium, and gold, although its use may not presently be very practical for plating gold. The presence of a substantial amount of ammonium ion has been found to be highly desirable in zinc plating baths from the standpoint of obtaining the highest quality deposits, but conventional anode leads are entirely unsatisfactory in the combined presence of chloride and ammonium ions. Since the silver-coated leads described herein are substantially unaffected by that combination, the present method is especially beneficially employed with such baths.

The plating conditions employed will depend primarily upon the composition of the specific bath used, and the proper selection thereof will be apparent to those skilled in the art. However, it will be appreciated that at elevated temperatures and with high current densities the problems of contamination and deterioration of the anode lead are normally proportionately more severe, and that the advantages of the invention will therefore be most apparent under the more severe conditions.

Except as is described herein, the present invention does not require any particular methods or techniques and, for example, it is applicable to both still or tank and barrel plating procedures. The term metallic workpiece includes any structure suitable for plating in a bath, which consists wholly or partially of an electricaly conductive metal. Thus, metal surfaced articles and articles containing metallic elements are encompassed as well as unitary metal structures.

With respect to the structure of the anode lead or conductor, any base metal which is a reasonably good conductor of electricity and which can be coated with silver may be employed; it will be appreciated that the present invention is directed to the use of metals for the base which are susceptible to attack by the chloride ion of the bath. Although copper is preferred due to its relatively low cost and high level of conductivity, other metals such as aluminum, brass and the like may be substituted therefor. Furthermore, in applications in which structural properties are of paramount importance, the base material may be one having high structural strength and relatively low conductivity, such as iron, since the silver coating provides a highly effective conductive path.

The metal base member of the conductor must be coated with a continuous nonporous layer of silver at least on the portion thereof which is to be immersed in the plating bath; however, it will normally be most convenient to employ conductors which are substantially completely coated with silver. The silver coating may be producted on the base by any suitable mean including flame-coating techniques, cladding, etc.. but most desirably the silver is present in the form of a relatively thin electroplated layer. From the standpoint of economy, the silver layer should be as thin as possible consistent with the production of a nonporous coating. For example, an electroplated deposit of one mil thickness has been found to be adequate to completely cover the base and provide the full advantages of the invention. There may, however, be instances in which it is desirable to apply a thicker coating of metal, such as to furnish resistance against abrasion or to provide improved conductivity. By use of the term coating of silver, silver alloys as well as pure silver are intended to be encompassed, so long as the alloys contain a sufficient proportion of silver to provide the protection from chloride ion which is desired. Preferably, the amount of silver in such an alloy will be about 90 percent by by weight or higher, and most desirably it is substantially pure, i.e., above 99.5 percent silver. The configuration of the anode lead may vary widely; it may simply be a silver-plated wire or a more elaborate structure designed for a specific application. For example, in the so-called strip plating of zinc the anode leads are relatively heavy copper members on which pure iznc anodes can be supported near the bottom of the plating tank, and the use of silvercoated copper members of this type are encompassed Within the scope of the present invention.

Although the theory underlying the invention is not fully understood, it is believed that during initial operation an insoluble film of silver chloride is formed on the outer surface of the silver coating. Such films appear to act as barriers to current flow, as a result of which dissolution of silver and hence of the underlying base metal in the bath is substantially prevented. As a result, the anode leads have a long operational life, and the bath and electrodeposits produced therefrom are substantially uncontaminated by the metal of the anode lead. For example, a copper anode lead plated with silver one mil in thickness may be maintained in continuous operation without replacement or repair for a period of six months or longer; in any event the lead can be readily repaired, if necessary, simply by electrodeposition or otherwise coating it with an additional small quantity of silver.

Exemplary of the eflicacy of the present invention are the following specific examples.

EXAMPLE 1 A plating solution was prepared by dissolving in water about 200 grams per liter of ammonium chloride, about 150 grams per liter of zinc chloride and about grams per liter of brightening and surface active agents; the pH of the bath was about 4.5. A thin strip of copper measuring one by three inches and plated with silver to one mil thickness was connected in parallel with a similarly dimensioned strip of zinc, and both strips were immersed in the zinc plating bath along with two steel test cell panels. The strips of silver-plated copper and of zinc were anodically connected through a common conductor to a rectifier, and the steel test panels were connected to the opposite pole thereof to serve as the cathode or workpiece. The temperature of the bath was maintained at about 100 Fahrenheit, and 2.5 volts were impressed across the anode and cathode to produce a current density of about 100 amperes per square foot over the total sur face of both strips. The silver-plated anode lost only 1.1 milligrams during the initial thirty-minute period of the test.

EXAMPLE 2 The bath employed was of the same composition as that used in the previous example, but the apparatus was modified to permit measurement of the total current and of the current passing through each of the anode strips. Thus, one side of an ammeter was connected to the rectifier and the other side connected to two additional ammeters in two parallel circuits, one of which included the silver-plated strip and the other of which contained the zinc strip. A total current of 8.4 amperes was passed through the bath at about Fahrenheit; initially 1.5 amperes flowed through the silver'plated panel, but the current gradually diminished and fell to zero in 1.5 minutes. In two hours time the weight of the plated strip decreased by only 2.5 milligrams; after an additional twohour period, the further weight loss was only 0.1 milligram. The absence of current flow during the test after the initial phase thereof shows that the weight stability of the plated panel was attributable to the presence of the protective layer and not simply to a change of form which would be accompanied by current flow.

- From Examples 1 and 2 it is readily seen that the silver-plated copper material provides an anode lead through which there is substantially no electricity conducted to the solution after the initial formation of a protective layer, and from which there is little weight loss and contamination. In both instances, the panels were removed, dried, and used in subsequent tests; the additional weight losses which resulted were negligible, and there was no detectable flow of current through the bath therefrom.

EXAMPLE 3 Using a bath of the same composition as that employed in Examples 1 and 2, a strip of silver-plated copper having two pieces of zinc mounted on both sides thereof was employed as the anode. The thickness of silver on the copper was once again about one mil, and the zinc was positioned on the strip so that only the edges and an additional limited portion of the silver surface were directly exposed to the solution; the ratio of exposed zinc to silver surface was about 6.0: 1.0.

Using a steel plating cell panel as the cathode, plating was effected by passing 4.2 amperes (equivalent to 100 amperes per square foot at the anode) through the solution for two hours at 100 Fahrenheit. At the end of the test, no weight change could be detected in the silverplated panel. The same test was repeated using a new anode of the same design, and passing 8.4 amperes (equivalent to 200 amperes per square foot at the anode) through the solution at 100 Fahrenheit. After 15 minutes, the silver-plated portion of the anode had lost 0.3 milligram, and only 0.4 additional milligram was lost during a subsequent three-hour period.

The apparatus employed in this example simulated that used for strip plating, in which the proportion of the lead exposed in the solution is relatively small compared to the anode surface area; the exposed portion of the silver-plated strip and the zinc strips act as the conductor and the anode, respectively. Under such circumstances, it can be seen that the plated strip functions admirably as a conductor with substantially no weight loss therefrom except during the initial phase of plating.

Thus, it can be seen that the present invention provides a method of electroplating metal from a chloride-containing bath in which the anode conductor is relatively inexpensive and is substantially insoluble to avoid contamination of the bath and of the electrodeposited coating. The lead to the anode is a highly efiicient conductor of electricity despite its inertness to attack; it may be produced relatively inexpensively and it has a long operational life.

Having thus described the invention, I claim:

1. In a method of electrodepositing metal upon a workpiece, the steps comprising: immersing an electrode and a metallic workpiece in a conductive solution of the metal to be deposited containing at least about 10 grams per liter of free chloride ion and ammonium ion, said metal being selected from the group consisting of zinc, nickel and cadmium, said electrode being of said metal and soluble in said solution under the conditions of operation; anodically connecting said electrode and cathodically connecting said workpiece to a source of DC. potential from said source across said anode and cathode to deposit said metal from solution upon said cathode, the conductor between said electrode and source of DC. potential being at least partially submerged in said solution and consisting essentially of an electrically-conductive metal base member and a substantially nonporous coating of silver thereon, the metal of said base member being other than silver and being susceptible to attack by chloride ion in acid 5 solution and said coating being present continuously over the entire surface of at least the portion of said conductor which is submerged in said solution, said coating developing a surface portion substantially nonconductive to said solution after initial operation of said method.

2. This method of claim 1 wherein said metal of said conductive base is copper.

3. The method of claim 2 wherein said coating exceeds about 99.5 percent silver.

tion is zinc.

5. The method of claim 1 wherein said solution contains about 10.0 to 250.0 grams per liter of zinc chloride to provide said metal to be deposited and about 20.0 to 220.0 grams per liter of ammonium chloride.

References Cited FOREIGN PATENTS 722,653 11/1965 Canada 204279 10 GERALD L. KAPLAN, Primary Examiner U.S. Cl. X.R. 

